Chelmsford Amateur Radio Society Advanced Course (10) Measurements Part-2 - Frequency

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Chelmsford Amateur Radio Society Advanced
Course(10) MeasurementsPart-2 - Frequency
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Frequency Measurement

• It is important to know the frequency that your
  receiver is tuned to, and the frequency of a
  signal radiated by a transmitter.
• In particular it is a requirement that the signal
  is within the operational amateur band.
• Do not assume that your equipment cannot operate
  outside the band.
• There are several means to either measure
  frequency or to calibrate a receiver or VFO.
• The syllabus requires you to have an
  understanding of all of these

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Frequency Standards

• Even when a crystal oscillator is used as a
  calibration source it is necessary to set it
  against a frequency standard.
• For a long time now national administrations have
  provided a service by transmitting ultra stable
  frequencies to allow accurate calibration of
  instruments.
• The transmissions can be found on one of the
  following frequencies
• 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, 20MHz and 25
  MHz.

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Absorption Wavemeter

• An absorption wavemeter consists of a calibrated
  tuned circuit, which absorbs power when the
  circuits are tuned to the same frequency as that
  being tested.
• The power collected can be made to move a meter
  or light a bulb.
• If the wavemeter is held close enough to a RF
  circuit the current induced will be high enough
  to light a bulb or move the meter.
• If the wavemeter has been calibrated it is easy
  to tune in to the frequency in question, peak for
  maximum and read it off.

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Crystal Calibrator

• The absorption wavemeter is invaluable for
  confirming circuits are tuned to the correct
  band, but they are not accurate enough for
  frequency measurements.
• Thus it is necessary to check against an accurate
  frequency standard.
• The simplest is a 100 kHz oscillator that
  produces markers across the bands.
• Another type is a highly stable 1 MHz oscillator
  with a divider circuit to produce 100 kHz and 10
  kHz markers.

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Heterodyne Wavemeter

• Before frequency counters this wavemeter
  represented one solution for measuring frequency.
  
• It provides crystal control frequency markers
  with a calibrated highly stable VFO to
  interpolate between the markers.
• Its use can be either
• To receive the unknown frequency and mix it with
  the oscillator to produce an audible output.
• Or to feed the unknown signal and the heterodyne
  frequency into a receiver, zero beat the pair,
  and then read the frequency from the meter scale.

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Digital Frequency Counter

• These can be purchased or constructed easily,
  they count a number of cycles over a given time
  period.
• The accuracy of a meter depends upon the internal
  clock signal which is derived from a crystal.
• This accuracy improves if the crystal is housed
  within a temperature controlled oven.
• From time to time this crystal should be checked
  against a standard frequency.
• The resolution of a instrument is the smallest
  digit displayed for the frequency range in
  operation and the clock frequency stability.

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Measurement Tips

• Measurement Tolerance.Is the percentage of the
  measured frequency and will vary from band to
  band. Allow for such errors when close to band
  edges.
• Calibration Accuracy.How well can you interpret
  your dial? In mechanical systems how good is the
  tuning mechanism? When setting the instrument how
  close can the reference oscillator be adjusted to
  the frequency standard.
• Frequency Drift.All oscillators drift with
  voltage and temperature. Reference oscillators
  must be well made, and be temperature or even
  oven controlled.
• Measurement Bandwidths.Care should be taken when
  measuring on wide bandwidth that you are in the
  centre. Always measure on a narrow bandwidth.